The present invention relates generally to the field of lithography and multi-layered flat or figured mirrors for use in lithography systems. More specifically, the present invention relates to a method of coating flat and figured optical substrates with high-reflectivity multi-layer coatings for use at Deep Ultra-Violet ("DUV") and Extreme Ultra-Violet ("EUV") wavelengths.
In general, lithography refers to processes for pattern transfer between various media. Lithography is a technique used for integrated circuit ("IC") fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a "transparency" of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the "projecting" radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Currently, there are a number of advanced lithography processes that use either steppers or proximity printers to produce state-of-the-art commercially-available products. Electron beam photolithography, on the other hand, is used almost exclusively by researchers developing ultra-small minimum feature size ("mfs") semiconductor processes, basic research, or low-volume radiation-hardened military integrated circuits. Use of electron beam photolithography for large-scale commercial applications such as semiconductor processing is not practical because the electron beam can damage the underlying circuit and the defect free masks have not been reproducible.
Proximity X-ray lithography uses a 1:1 transmission mask in conjunction with X-ray radiation to project a pattern onto a photoresist. The very short wavelength of the X-ray radiation results in the possibility for very high resolutions, e.g., mfs less than or equal to 0.1-.mu.m. However, the 1:1 transmission masks required for this technology are extremely difficult to fabricate and maintain. The lack of a projection (demagnification) process means that features on the mask, must be the same size as the desired feature size on the wafer (0.1-.mu.m). In addition, proximity lithography suffers from other problems such as distortion of a thin membrane mask, misalignment between the mask and wafer, and resolution degradation caused by secondary photoelectrons radiating from the silicon base plate.
Electron beam and ion projection lithographies use electrons and ions respectively to project a pattern onto a photoresist. The particles have very high energies and are able to efficiently expose the photoresist. The particles are focused using electromagnetic lenses similar to those used in transmission electron microscopes. Presently, both technologies suffer from technical problems that must be overcome for the technology to become viable. Electron beam and ion projection lithographies require new column designs to minimize the column interaction and space charge impact on resolution. In addition, both technologies require the development of membrane masks with the appropriate magnification factor.
Phase shifting reticle projection photolithography technology uses special masks and coherent laser light to produce images for a mfs on the order of 0.1-.mu.m. This process requires high power light and long exposure times making the photolithography process susceptible to vibrations. The vibrations can introduce distortions into the photoresist which result in defects in the IC and consequently reduce the yield of the photolithography process. In addition to a reduced product yield, phase shifting projection photolithography systems are expensive compared to traditional projection lithography systems. However, the phase shifting projection photolithography systems are preferred to electron beam photolithography systems because they are less expensive, safer, and require less space in a cleanroom. Because of the high cost, low yield, low reliability, and low volume throughput, phase shifting projection photolithography is used primarily by researchers developing ultra-small mfs semiconductor processes and basic research. These systems suffer from fundamental limitations that restrict their usefulness in the volume production photolithography market.
Present DUV photolithography systems use refractive optical systems (lenses) with 248-nm radiation to create mfs of less than or equal to 0.4-.mu.m. As future generation photolithography systems move to progressively shorter wavelengths, the performance and lifetime of lenses will decline as a result of increased optical absorption of the lens materials at these shorter wavelengths. The absorption is already problematic in lenses designed for use at 193-nm in next generation photolithography systems. SiO.sub.2 lenses suffer from absorption which leads to optical compaction while CaF.sub.2 lenses suffer from stress-induced birefringence. Lenses will be completely useless for subsequent generation photolithography systems designed for use at even shorter wavelengths (&lt;193-nm). Multi-layered, high-reflectance coatings (mirrors) represent the only viable alternative optical elements for the creation of high-speed, reliable, reasonably-priced photolithography systems for use at DUV and EUV wavelengths (193-nm to 13.4-nm).
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits as a function of the wavelengths of the optical radiation. A recognized way of reducing the feature size of circuit elements is to lithographically image them with radiation of a shorter wavelength. The use of EUV radiation (a.k.a., "soft" X-rays) with wavelengths less than or equal to 100-nm is now at the forefront of research in an effort to achieve the desired smaller feature sizes. Indeed, the National Technology Roadmap for Semiconductors predicts that, by the year 2010, the mfs will decrease from about 0.35-.mu.m to 0.07-.mu.m. Mass production of large-area microchips with feature sizes as small as 0.10-.mu.m is contemplated with the use of EUV radiation, e.g., at a wavelength of 13.4-nm. EUV radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that most materials are extremely absorptive and do not make useful lenses at EUV wavelengths. In addition, single surface reflectors (mirrors) offer minimal reflectivity because their refractive indices, N (N=n+ik), have a real component, n, that approaches 1.00 and an imaginary component, k (a.k.a., absorption constant), that is very high for these short wavelengths.
The present state-of-the-art projection lithography for Very Large Scale Integration ("VLSI") is a 16 megabit microchip with circuitry built to design rules of 0.5-.mu.m mfs. Efforts directed to further miniaturization take the initial form of more fully utilizing the resolution capability of presently-used ultraviolet delineating radiation. Deep Ultra-Violet ("DUV") radiation, wavelength range of 100-nm to 300-nm, with techniques such as phase masking, off-axis illumination, and step-and-repeat, can permit design rules (minimum feature size or space dimension) of 0.25-.mu.m or slightly smaller. Mass production of large-area microchips with feature sizes as small as 0.1-.mu.m is contemplated with the use of EUV radiation, e.g., at a wavelength of 13.4-nm.
Two EUV radiation sources are under consideration: a laser plasma source and a synchrotron source. Additionally, a variety of EUV (X-ray) patterning approaches have been considered. Probably the most developed form of X-ray lithography is proximity printing. In proximity printing, the object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing or distance from the wafer (i.e., out of contact with the wafer, thus, the term "proximity"), which lessens the likelihood of mask damage but does not eliminate-it. Making perfect masks on a fragile membrane continues to be a major problem. The necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelicity in the incident radiation. Radiation of wavelength .lambda..ltoreq.1.6-nm is required for 0.25-.mu.m or smaller patterning in order to limit diffraction at feature edges on the mask.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is greatly reduced. This reduces the costs associated with replacement masks and equipment downtime. Projection lithography enables the use of imaging or camera optics in-between the mask and the wafer which compensate for edge scattering or diffraction and, so, permit use of longer wavelength radiation. The resulting system is known as extreme ultra-violet lithography, or EUVL, a.k.a., soft X-ray projection lithography ("SXPL").
Hawryluk et al., U.S. Pat. No. 5,003,567, describe an approach to soft X-ray projection lithography ("SXPL"), which also takes advantage of recent advances in the field of EUV optics. For example, it is possible to build an EUV reduction camera using curved imaging mirrors, which may be spherical or aspherical. Each mirror includes a substrate of a material such as glass-ceramic or sintered glass having a low coefficient of thermal expansion. The first surface of the substrate is typically ground to high precision and polished, which is then overcoated with a multi-layer coating. The alternating (i.e., paired) materials have a large difference in complex index of refraction at the EUV wavelength being used. As a consequence of the periodic variation of complex index of refraction, the mirror exhibits high EUV radiation reflectivity at certain angles of incidence. A typical EUV reduction camera uses a reflective mask consisting of a thin, IC metallization pattern overlying an EUV-reflective, multi-layer coating on a polished (flat or curved) substrate. The mask is positioned such that EUV radiation incident thereupon is reflected from the mask onto a primary mirror, then onto one or more secondary mirrors, and from the last secondary mirror onto a wafer surface. Image reductions as great as 20:1 have been achieved in this manner. A disadvantage of the Hawryluk et al. design, however, is that it requires a curved, pre-distorted object mask, which is difficult to fabricate accurately and requires the optics to be adapted accordingly.
Available refractive indices are still quite close to one at .lambda..ident.10-nm, but are sufficient to permit fabrication of multi-layer reflective optics or Distributed Bragg Reflectors ("DBR"). DBR optics resulting in approximately equal to 60 to 65% reflectance for use with 14-nm radiation have been constructed and used to obtain 0.1-.mu.m feature sizes. This approach, providing for full-feature (non-scanning), reduction projection is severely limited by field curvature. While needed resolution is obtainable, field size is very small, e.g., 25.times.50 -.mu.m with a feature size of 0.1-.mu.m.
Present photolithography systems use refractive optical systems (lenses) to focus the light and operate at 248-nm radiation (DUV) to create a mfs of less than 0.4-.mu.m. As future generation photolithography systems move to progressively shorter wavelengths, the performance and lifetime of lenses will decline as a result of increased optical absorption of the lens materials at these shorter wavelengths. The absorption is already problematic in lenses designed for use at 193-nm in next generation photolithography systems such as SiO.sub.2 (silica) and CaF.sub.2 (fluorite). SiO.sub.2 lenses suffer from absorption, which leads to optical compaction, while CaF.sub.2 lenses suffer from stress-induced birefringence.
SiO.sub.2 has an absorption edge approaching 193-nm that causes the lenses to absorb a small but significant portion of the incident radiation. The radiation is converted to heat energy which can cause the SiO.sub.2 lens material to recrystallize and undergo optical compaction. The radiation conversion changes the optical properties of the lenses in a significant and permanent manner, drastically altering the focusing capabilities of the optical system. In addition, absorption of the incident radiation results in the formation of defect centers which are highly absorptive themselves. This increase in absorption causes a progressive reduction in transmission as a result of the formation of additional defect centers and a serious degradation of the focusing capabilities as a result of enhanced optical compaction.
CaF.sub.2 is less susceptible to these problems because it is less absorptive than SiO.sub.2 at 193-nm. However, lenses made from CaF.sub.2 suffer from a number of other problems that limit their usefulness. Optical grade CaF.sub.2 is extremely expensive, especially for the larger-sized focusing elements required for photolithography optical systems. CaF.sub.2 lenses also suffer from localized variations in the refractive index as a result of stress-induced birefringence effects. Localized heating as a result of even moderate absorption or mechanical strain as a result of the weight of the lens element itself may be sufficient to cause significant birefringence in the lens. This significantly degrades the performance of each lens and will limit the useful lifetime of the optical system.
The amount of absorption for both SiO.sub.2 and CaF.sub.2 lenses increases as the operating wavelength of the optical system is reduced further. In addition, single-surface mirrors become impractical at progressively shorter wavelengths as a result of decreased reflectance. For example, aluminum-coated mirrors offer only 92% reflectance at 193-nm. While 92% reflectance seems like a reasonable high-reflectance value, the total loss in light intensity in an optical system containing, for example, four mirrors will be about 28.4%. The total loss in light intensity is even greater for shorter wavelengths. Therefore, the use of lenses or single-surface mirrors is completely impractical at progressively shorter wavelengths. On the other hand, multi-layered mirrors can now be fabricated that offer greater than 99% reflectance at 193-nm. This results in a total loss in light intensity of only 3.9% for a comparable optical system with four mirrors. For this reason, it is best to design and fabricate highly-reflective multi-layered mirrors for use as focusing elements at these short wavelengths (e.g., 193-nm and 13.4-nm) as in the present invention. Multi-layered, high-reflectance coatings (mirrors) represent the only viable alternative optical elements for the creation of high speed, reliable, reasonably-priced photolithography systems for use at DUV wavelengths such as 193-nm and EUV wavelengths such as 13.4-nm.
Referring to FIGS. 1a and 1b, one technique for coating figured optics, which was developed at AT&T Bell Laboratories, uses a contoured, shaped baffle 10 ("uniformity mask") placed directly in front of the rotating figured optic to control the relative deposition rate across the optic, thus controlling the multi-layer coating thickness profile. See also D. L. Windt and W. K. Waskiewicz, Multilayer Facilities Required for Extreme-Ultraviolet Lithography, J. Vac. Sci. Technol. B, 12(6), pp. 3826-3832 (November/December 1994). The shape of the baffle 10 determines the relative deposition rate as a function of position on the optic 15. The appropriate shape of the baffle 10 is determined empirically and requires the deposition and characterization of numerous test mirrors. Any change in the curvature of the optic or the deposition parameters requires yet another set of calibration experiments to redesign the baffle 10. This process is very cumbersome, non-manufacturable, and expensive. Additionally, the effective shape of the baffle 10 changes as it accumulates thin film material from the sputter deposition process over time, which changes the resultant deposition profile on the figured optic 15 over time. Effectively, the use of the baffle deposition process is not practical for commercial applications.
Researchers at Lawrence Livermore National Laboratories have developed a process for coating EUV mirrors in which a rotating figured optic is scanned in a circular path around the entire deposition chamber. S. P. Vernon, M. J. Caroy, D. P. Gaines, and F. J. Weber, Multilayer Coatings for the EUV Lithography Front-End Test Bed, OSA Proceedings on Extreme Ultraviolet Lithography, Monterey, Calif., vol. 23, pp. 33-40 (Sep. 19-21, 1994). The optic is continuously rotated at a constant velocity and moved with respect to the sputter source. The effective deposition rate at any point along the circular path is affected by the source-to-optic distance. By changing the velocity of the optic at various points along the circular path, it is possible to control the resultant thickness profile. The thickness is built up at the edges of the optic by reducing the velocity of the optic at certain positions with respect to the target, which can be modeled mathematically. This process has been used successfully to coat a limited number of figured EUV mirrors.
It is expected that effort toward adaptation of EUV sources and optics for EUVL will continue. Multi-layered, high-reflectance coatings (mirrors) represent the only viable alternative optical elements for the creation of high speed, reliable, reasonably priced photolithography systems for use at DUV and EUV wavelengths. Currently, there is no manufacturable process for coating the figured DUV and EUV mirrors necessary for advanced photolithography systems. As such, the present invention discloses a system and method of coating, for example, flat or figured optical substrates, with a high-reflectivity multi-layer coating that will permit the development and implementation of projection EUV photolithography production in the mass commercial micro-electronic marketplace. The present invention presents for the first time a manufacturable-controlled and repeatable--deposition technique for fabricating DUV and EUV mirrors. The ability to precisely coat high-reflectance, flat or figured DUV and EUV optics offers a substantial reduction in the minimum attainable feature size through the use of shorter wavelength radiation, which results in substantial miniaturization of ICs, sensors, and other micro-electronic components.